How Salt Can Kill

People with cystic fibrosis have a genetic defect that leaves them unable to absorb salt properly into epithelial cells, including the ones that line the lungs. What kills them, however, is not the excess salt itself but a lung-clogging mucus that becomes a breeding ground for airborne bacteria. Researchers had never been able to figure out how the salt transport problem led to the mucus--until last March, when pediatric allergist Jeffrey Smith and his colleagues from the University of Iowa College of Medicine in Iowa City announced that they’d found the missing link.

Smith’s team discovered that an antimicrobial agent produced by the lung’s epithelial cells loses most of its punch in cf patients. Normally this in-house antibiotic quietly dispatches bacteria that enter the lungs. We found that it is salt-sensitive, says Smith. If the salt concentration is high, it fails to kill bacteria.

Because the salty lungs of cf patients disable the natural antibiotic, Smith explains, their immune systems are forced to launch a full-scale response to any bacteria that drift in. White blood cells kill most of the bugs, but the battle is messy and never-ending. The immune cells inflame the lung’s delicate branching airways, and their spent bodies contribute to the formation of the sticky, hard-to-break-up mucus. Because it’s also warm and rich in nutrients, the mucus is the perfect growth medium for more bacteria. And so the cycle of infection and inflammation continues.

The Iowa team’s discovery may lead to new treatments. Maybe we can provide more of the stuff that can kill bacteria, muses Smith. Or maybe we could modify it so that it’s no longer salt-sensitive. There’s a lot of potential here.